Integrated circuits (ICs) form the basis for many electronic systems. Essentially, an integrated circuit (IC) includes a vast number of transistors and other circuit elements that are formed on a single semiconductor wafer or chip and are interconnected to implement a desired function. The complexity of these integrated circuits (ICs) requires the use of an ever increasing number of linked transistors and other circuit elements.
Many modern electronic systems are created through the use of a variety of different integrated circuits; each integrated circuit (IC) performing one or more specific functions. For example, computer systems include at least one microprocessor and a number of memory chips. Conventionally, each of these integrated circuits (ICs) is formed on a separate chip, packaged independently and interconnected on, for example, a printed circuit board (PCB).
As integrated circuit (IC) technology progresses, there is a growing desire for a “system on a chip” in which the functionality of all of the IC devices of the system are packaged together without a conventional PCB. Ideally, a computing system should be fabricated with all the necessary IC devices on a single chip. In practice, however, it is very difficult to implement a truly high-performance “system on a chip” because of vastly different fabrication processes and different manufacturing yields for the logic and memory circuits.
As a compromise, various “system modules” have been introduced that electrically connect and package integrated circuit (IC) devices which are fabricated on the same or on different semiconductor wafers. Initially, system modules have been created by simply stacking two chips, e.g., a logic and memory chip, one on top of the other in an arrangement commonly referred to as chip-on-chip structure. Chip-on-chip structures most commonly use micro bump bonding technology to electrically connect the working surfaces of two chips. Several problems, however, remain inherent with this design structure. For example, this approach is limited in the number of chips that can be interconnected as part of the system module.
In the past several years, multi-chip module (MCM) technology has been utilized to stack a number of chips on a common substrate to reduce the overall size and weight of the package, which directly translates into reduced system size. Existing MCM technology is known to provide significant performance enhancements over single chip or chip-on-chip (COC) packaging approaches. For example, when several semiconductor chips are mounted and interconnected on a common substrate through very high density interconnects, higher silicon packaging density and shorter chip-to-chip interconnections can be achieved. In addition, low dielectric constant materials and higher wiring density can also be obtained which lead to the increased system speed and reliability, and the reduced weight, volume, power consumption and heat to be dissipated for the same level of performance. However, existing MCM approaches still suffer from additional problems, such as bulky package, wire length and wire bonding that gives rise to stray inductances that interfere with the operation of the system module.
Until most recently, the most promising interconnect technology that is still within the confines of research but is close to the ideal high-performance “system on a chip” is the three-dimensional (3D) wafer-to-wafer vertical stack integration. Whereas MCM technology seeks to stack multiple chips on a common substrate, 3-D wafer-to-wafer vertical stack technology seeks to achieve the long-awaited goal of stacking many layers of active IC devices such as processors, programmable devices and memory devices inside a single chip to shorten average wire lengths, thereby reducing interconnect RC delay and increasing system performance. In direct 3-D integration, active device wafers are bonded together, while all active layers are electrically interconnected using vertical vias.
One of the major challenges of 3-D wafer-to-wafer vertical stack integration technology is the metal bonding between wafers and between die in a single chip. In general wafers are bonded one at a time in a bond chamber with pressure and heat applied through a standard rigid press. However, typical wafers to be bonded may have thickness variations in a metal bonding layer which when pressed together under constant pressure will prevent good contact and instead, make contact at only a few high points along the wafers. Therefore, it is desirable to allow for thickness variations in the metal bonding wafers and to ensure that wafers are bonded more uniformly and effectively.